1. Field of the Invention
The present invention relates to an indolylphthalide compound and a thermal recording material using the same.
2. Description of the Related Art
Recording materials using an electron-donating dye precursor and an electron-accepting compound have been well known as pressure-sensitive papers, heat-sensitive papers, photosensitive/pressure-sensitive papers, conductive thermal recording papers, thermal transfer papers or the like. Detailed information on the recording materials is disclosed, for example, in U.K. Patent No. 2,140,449, U.S. Pat. Nos. 4,480,052 and 4,436,920, Japanese Patent Application Publication (JP-B) No. 60-23992, Japanese Patent Application Laid-Open (JP-A) Nos. 57-179836, 60-123556 and 60-123557, and the like.
In recent years, serious studies have been conducted on improvements to a recording material with respect to image density, color fastness of an image portion, light fastness of a white portion, and the like. Various examples of known electron-donating dye precursors that form images in colors ranging from blue to cyan include triphenylmethanephthalide-based compounds, phenothiazine-based compounds, indolylphthalide-based compounds and the like.
Among indolylphthalide-based compounds, some examples of a so-called indolyl(aza)phthalide-based compound have been known that are obtained by substituting a nitrogen atom for a carbon atom on a benzene ring in the phthalide moiety. Concrete examples thereof are detailed in JP-B No. 61-4856, JP-A Nos. 61-168664, 61-291654 and 62-270662, and the like.
Although the compounds described above form images in colors ranging from blue to cyan with good density, they have problems in that they satisfy neither of performances of good color fastness of an image upon exposure to light and high color development performance. Furthermore, a conventional indolylphthalide compound has an absorption band having a gradual slope on the short wavelength side of the first absorption band. The gradual slope causes a developed color image to have a low chroma.